1. Field of the Invention
The invention relates generally to the field of measurement of fluid properties using x-ray radiation. More particularly, the invention relates to x-ray radiation methods and apparatus for determining fluid phase fractions of multiple phase fluids wherein gas comprises a majority of the fluid under examination.
2. Background Art
Wellbores are drilled through subsurface Earth formations for the purpose of extracting useful fluids from the subsurface formations, such as petroleum. Typically, when formation fluid is extracted from the formations and is moved into a wellbore, it consists of a mixture of various fractions of oil, gas, and water. Certain well operations, for example, include pumping fluid such as water, natural gas or carbon dioxide into a wellbore drilled through the same formations and adjacent to a fluid-producing wellbore to help force formation fluid from the formations into the fluid-producing wellbore. A phase fraction meter is useful in such instances to dispose in the fluid-producing wellbore to show when unwanted fluids, such as water, carbon dioxide or natural gas are being extracted from a particular subsurface formation along with the desired fluid, such as oil. Information concerning the fractional volume of particular fluids being extracted from a wellbore is also useful in optimizing the production of fluid from a subsurface reservoir. The gas to liquid ratio, and the oil to water ratio, for example, are important and constant monitoring of such ratios can assist in determining the best utilization of a reservoir.
Additionally, fluid that is high fractional volume of water will be of less monetary worth than fluid having a high fractional volume other components, such as gas and oil. By determining the fractional volumes of each of gas, oil and water early in the process of extracting fluid from a reservoir, it is possible to estimate the economic value of any given operation affecting fluid production from a subsurface reservoir. By testing fluid when delivered from an unknown source, it is also possible for a buyer of produced fluids to determine if the fluid fractions represented by the fluid producer are what are in fact being provided by the fluid producer.
One approach for determining fluid fractional composition known in the art includes a separator or a large tank used to physically store some amount of fluid extracted from a well. Segregating the various fluid components is performed in the tank through a gravity-based process. Such process requires stable conditions inside the separator, and the results may take an extended period of time to obtain. The required degree of condition stability may be difficult or even impossible to obtain, and because of the extended time needed to obtain fluid component separation such technique may create an obstacle to economic recovery of hydrocarbons because flow from the wellbore must be stopped during the testing process. Separator-based systems can also provide erroneous results when there is some commingling of the various fluid components (“phases”). Additionally, viscous fluids such as heavy oil make accurate separation and testing difficult.
Other systems known in the art may allow for substantially “real-time” phase fraction determination using a radiation source and detector. Such fractional composition measuring devices use chemical isotope radiation sources and may be deployed for long periods of time in unattended locations. The locations often are not secure and may encounter variable ambient environmental conditions. Security and environmental risks associated with chemical isotope radiation sources makes it desirable to use non-chemical radiation sources for fluid fractional composition measuring devices. Electrical radiation generators would alleviate some of the foregoing concerns, but most electrical radiation generators (such as x-ray generators) have radiation output that is related to the degree of accuracy with which actuation voltage and target current can be controlled. As a result there are certain benefits to the use of chemical sources. Specifically, the change of their output radiation over time is stable, enabling such sources to provide a highly predictable radiation flux.
An example of using an electrically powered x-ray generator for fractional fluid volume composition determination is described in U.S. Pat. No. 5,689,540 issued to Stephenson et al. and assigned to the assignee of the present invention. The device described in the Stephenson et al. '540 patent is a system for imparting a spectrum of photon radiation through a fluid sample and determining a fluid fraction by analysis of detected radiation after it has passed through the fluid sample.
Another device is described in U.S. patent application Ser. No. 11/425,285 filed on Jun. 20, 2006, entitled, “HIGH PERFORMANCE X-RAY MULTIPHASE FRACTION METER” and assigned to the assignee of the present invention. Such device includes an x-ray generator with a special filter disposed in the output of the x-ray generator. Filtered x-rays from the generator are detected both at a reference detector, which is essentially disposed directly in the radiation output of the generator, and a measurement detector, which is disposed along a radiation path through a chamber in which a sample of the fluid under analysis is disposed. Fractional volumes of three different phases, gas, oil and water may be determined by analysis of the radiation detected by the measurement detector. Measurements from the reference detector are used to automatically control various operating parameters of the x-ray generator so that analysis of the measurement detector signal is relatively unaffected by changes in x-ray generator output. The system disclosed in the foregoing patent application is susceptible to improvement in accuracy under conditions that include high fractional volume of gas, e.g., 90 percent or more gas volume fraction, in the fluid being examined. Accordingly, it is desirable to have a multiple phase fluid fraction analysis device that has improved accuracy under conditions of high fractional volume of gas.